JSFiddle - React, Tailwind, and code Playground

HTML

<div class="f">
  
  <aside>
    <div></div>
  </aside>
  <section>
    
  </section>
  
</div>

SCSS

.f{
  float: left;
  width: 100%;
}
aside{
  float: left;
  width: 30%;
  height: 500px;
  div{
    width: 100%;
    height: 300px;
    background: red;
  }
}
section{
  width: 70%;
  float: right;
  background: #fff;
  height: 500px;
}